1. Field of the Invention
The present invention relates to avalanche photodiodes and photon counting technology.
2. Description of Related Art
Avalanche Photodiodes (APD) are well known photosensitive devices used to convert optical signals into electrical signals. As such, APD""s behaves like standard photodiodes, as both APD""s and photodiodes convert optical energy into electrical signal. However, APD""s additionally incorporate a gain mechanism internal to the device itself, making it more sensitive. That is, while in a conventional p-i-n photodiode an individual photon is converted into one electron-hole pair, in an APD for each individual photon absorbed multiple electron-hole pairs are generated. This multiplication, however, introduces unwanted noise to the APD""s output. Therefore, there""s a constant effort by APD researchers and manufacturers to produce a sensitive, but reduced-noise, APD.
FIG. 1 depicts one possible structure of an APD 100 in a somewhat simplified form. While the depicted example APD 100 of FIG. 1 is in an etched mesa-form, the entire discussion herein is equally applicable to APD of the bulk-planar form. The APD 100 comprises a p-InP substrate 110; a p-InP buffer layer 120 and an n-InP layer 130, forming the wide-bandgap multiplication region; an n-InGaAsP grading or bandgap-transition layer 140 of an intermediate bandgap; and an n-InGaAs narrow-bandgap absorption layer 150. The intermediate-bandgap transition layer 140 is generally provided in order to reduce accumulation of charges at the interface between the multiplication and absorption regions, 130 and 150, respectively. Layers 105 and 115 are contacts, which can be made of, for example, AuInZn or AuSn. In this example, photons hxcexd are collected from the substrate side.
The example APD 100 depicted in FIG. 1 is of the separate absorption and multiplication (SAM) APD type. That is, in order to obtain high sensitivity to infrared light, the APD absorption region is built using narrow-bandgap InGaAs material 150. Using a wider bandgap material such as InP for the absorption region would not result in the APD having comparable infrared sensitivity. Similarly, in order to obtain adequate gain properties in the APD multiplication region 130, the multiplication material is optimally a wide-bandgap semiconductor, in this example InP, that is able to support the high electric fields needed to achieve charge multiplication without, at the same time, creating excessive unwanted carriers through an electric field-assisted method known as tunneling. In this manner, the photogeneration of carriers takes place in a material optimized for absorption and not in the multiplication region. Lastly, because SAM APDs comprise two semiconductor materials with distinct bandgaps, one or more grading layers 140 of intermediate bandgap materials are used to prevent trapping of charged carriers that would otherwise occur at the heterointerface between the dissimilar regions 130 and 150.
The multiplication noise of an APD has been generally shown to be a function of k, the ratio of hole to electron ionization constants within the multiplication medium of the APD, i.e., k=xcex2/xcex1. Note, however, that in some publications k is provided in terms of electron to holes ionization constants, i.e., k=xcex1/xcex2. However, unless specifically noted otherwise, in this disclosure the convention k=xcex2/xcex1 applies. In a series of papers, McIntyre et al., demonstrated that to improve the APD""s performance, one needs to achieve as low k value as possible. For example, they showed that an APD having k value approaching 1 would have a low gain-bandwidth product, whereas an APD having low values (k much less than 1) would have high gain-bandwidth product.
McIntyre, R. J., Multiplication Noise in Uniform Avalanche Diodes, IEEE Transaction on Electron Devices, ED 13, 164-168 (1966); McIntyre R. J., A New Look at Impact ionizationxe2x80x94Part I: A Theory of Gain, Noise, Breakdown Probability, and Frequency Response, IEEE Transaction on Electron Devices, 46, 1623-1631 (1999); Yuan, P., Anselm, K. A., Hu, C., Nie, H., Lenox, C., Holms, A. L., Streetman, B. G., Campbell, J. C., and McIntyre, R. J., A New Look at Impact Ionizationxe2x80x94Part II: Gain and Noise in Short Avalanche Photodiodes, IEEE Transactions on Electron Devices, 46, 1632-1639 (1999).
These works led researchers on a quest to discover low-k materials and structures for use in APD multiplication regions. For example, Campbell et al., have demonstrated that noise and gain-bandwidth performance can be significantly improved by utilizing very thin multiplication regions. They noted that InP has approximately equal hole and electron ionization rates (i.e., k≅1) and that, therefore, InP APD""s have high multiplication noise. They proposed an APD having a thin (200 nm-400 nm) In0.52Al0.48As multiplication region; demonstrated to result in k=0.18. They also noted, however, that thinning the multiplication region must be accompanied by an increase in the carrier concentration in the multiplication region. Otherwise, electric field in the narrow-bandgap absorbing layer would be too high and tunneling will ensue, leading to excessive dark current.
Campbell, J. C., Nie H., Lenox, C., Kinsey, g., Yuan, P., Holmes, A. L., Jr. and Streetman, B. G., High Speed Resonant-Cavity InGaAs/InAlAs Avalanche Photodiodes, IEEE Journal of High Speed Electronics and Systems 10, 327-337 (2000); Campbell, J. C., Chandrasekhar, S., Tsang, W. T., Qua, G. J., and Johnson, B. C., Multiplication Noise of Wide-Bandwidth InP/InGaAsP/InGaAs Avalanche Photodiodes, Journal of Lightwave technology 7, 473-477, (1989); Kinsey, G. S., Hansing, C. C., Holmes, A. L. Jr., Streetman, B. G., Campbell, J. C., and Dentai, A. G., Waveguide In0.53Ga0.47Asxe2x80x94In0.52Al0.48As Avalanche Photodiode, IEEE Photonics Technology Letters 12, 416-418 (2000); Kinsey, G. S., Campbell, J. C., and Dentai, A. G., Waveguide Avalanche Photodiode Operating at 1.55 m with a gain-Bandwidth Product of 320 GHz, IEEE Photonics Tachnology Letters 13, 842-844 (2001).
APDs can be operated in two regimes: the linear regime and the breakdown regime, the latter often referred to as Geiger mode. In the linear regime, the APD is biased below its breakdown voltage, and the output photocurrent of the APD is proportional to the intensity of light striking the absorption region 150 and to the APD gain that occurs in the multiplication region 130. In the Geiger mode of operation, the APD is biased above its breakdown voltage. In this mode of operation, a single photon can lead to an avalanche breakdown resulting in a detectable current running through the device, which thereafter remains in a conductive state. Consequently, the amplitude of the output signal in Geiger mode is constant and is not proportional to the number of photons absorbed. However, Geiger mode enables using APD""s for single-photon detection applications.
Among the various utilities, APD""s are used for single photon detection. Various applications require accurate detection of single photons. Among such applications is the detection of photon emission generated by switching semiconductor devices. Detection of such emission can be used to test, debug, and characterize the operation of such devices, especially in integrated circuits (IC""s). One system that can be used to detect such emission is described in U.S. patent application Ser. No. 09/995,548, commonly assigned to the assignee of the subject application, and which is hereby incorporated herein by reference in its entirety. Other systems are described in, for example, U.S. Pat. Nos. 4,680,635; 4,811,090; 5,475,316; 5,940,545; 5,208,648; 5,220,403; and Khurana et al., Analysis of Product Hot Electron Problem by Gated Emission Microscope, IEEE/IRPS (1986); all of which are incorporated herein by reference in their entirety.
As can be gathered from the above-cited references, much effort is being spent in investigating improvements to APDs in order to improve its utilization in various applications, including single photon detection.
The present invention provides an improved APD structure and an improved manner of operating APDs, particularly beneficial for single photon detection applications. The present invention is based on the realization that while the prior art teaching is to reduce the k value as much as possible so as to minimize noise generated by the APD and improve their operating bandwidth, an APD having a k value substantially equal to unity is actually advantageous for single photon detection applications. Thus, in one aspect of the invention, an APD for single photon detection is provided wherein the APD is structured so as to have a high k value, e.g., approximately 1.
In one example, an APD is provided having an absorption region and a separate multiplication region, wherein the multiplication region has a ratio of hole to electron ionization constants, i.e., a k value, of approximately 1. In one specific example, the multiplication region comprises a doped InP layer. In other examples, the multiplication region is made of a material having a k value approaching 1; for example, Ga0.18In0.82As0.39P0.61 having k=0.82.
In one aspect of the invention, an APD is provided having an absorption region; an intermediate bandgap transition layer; a field control layer; and a multiplication region having k≅1. The field control-layer is designed so as to produce an electric field reduction therein so as have the electric field over the absorption region at about 0-10V/xcexcm while maintaining the electric field over the multiplication region in excess of its breakdown field.
In another aspect of the invention, an APD is provided having an absorption region; a multiplication region; an intermediate bandgap transition layer; and a field control layer. The field control layer is designed so as to produce a reduction of electric field that is equal to the multiplication region""s breakdown electric field, plus or minus half the absorption region""s tunneling onset field. The tunneling onset field is defined herein as the value of field that causes excessive tunneling, thereby causing an unacceptable level of dark current. The tunneling onset field is a strong function of the bandgap within the absorption region. Campbell et al., Id., used the FIG. 20V/xcexcm for an In0.53Ga0.47As absorption layer. For various embodiments of the invention, the inventors have found the tunneling onset field for an InGaAs absorption layer to be about 10V/xcexcm.
The reason the field control layer is designed to produce a reduction in electric field equal to multiplication region""s breakdown electric field, plus or minus half the absorption region""s tunneling onset field, is twofold. First, the doping of the field control layer is typically imprecise during device manufacture, and this provides a tolerance to the manufacturing process. Second, during operation of the APD in the Geiger mode, the electric field within the multiplication region will be raised above the breakdown field in order to enable avalanches to occur. During such times, the electric field in the absorption region, which increases by the same value as the field in the multiplication region, must remain positive in order to remain photosensitive, and must not be allowed to exceed the breakdown field, in order to prevent excessive tunneling dark noise.
In yet another aspect of the invention, an APD is provided having an absorption region; a multiplication region; an intermediate bandgap transition layer; and a field control layer. The field control layer is doped with a dopant concentration designed so as to produce a reduction of electric field over the thickness of the field control layer that is equal to the multiplication region""s breakdown electric field, plus or minus 5V/xcexcm.
In another aspect of the invention, an APD is provided having an absorption region; a multiplication region; an intermediate bandgap transition layer; and a field control layer. The field control layer is designed so that the product of its dopant concentration and its thickness produces a reduction of electric field that is equal to the multiplication region""s breakdown electric field, plus or minus about 5V/xcexcm.
In another aspect of the invention, an APD is provided having an absorption region; a multiplication region; an intermediate bandgap transition layer; and a field control layer. The field control layer is designed so as to produce a reduction of electric field that, together with the field reduction over the multiplication region, causes a total field reduction that is equal to the multiplication region""s breakdown electric field, plus or minus the absorption region""s tunneling onset field.
In a further aspect of the invention, a method is provided for efficient detection of single photons, the method comprises constructing an APD having an absorption region; a multiplication region; and a field control layer. The method further comprises doping the field control layer so as to produce an electric filed reduction that is equal to the multiplication region""s breakdown voltage plus or minus about 5V/xcexcm. The method further comprises applying a potential across the APD so as to induce an electric field across the multiplication region that exceeds the breakdown field.
In yet a further aspect of the invention, a system is provided for detection of photons emitted from a semiconductor device. The system utilizing an APD detector having an absorption region; a multiplication region; and a field control layer. The field control layer is designed so that the product of its dopant concentration and its thickness produces a reduction of electric field that is equal to the multiplication region""s breakdown electric field, plus or minus about 5V/xcexcm.
Other aspects and features of the invention will be apparent to those skilled in the art from the description provided herein below, with reference to the appended drawings.